'Dead' galaxies are not so dead after all

May 30, 2011

Individual young stars and star clusters in the 'dead' elliptical galaxy, Messier 105, detected using the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). Messier 105 can be seen in the top, left corner, in an image from the Sloan Digital Sky Survey (SDSS; Data Release 8). The outlined region in the center of Messier 105 is expanded to reveal Hubble's unique view of the inner region of Messier 105, which is further expanded to unveil several individual young stars and star clusters (denoted by dashed circles; top, right). These signposts of recent star formation are unexpected in old, 'dead' galaxies. Data from HST's WFC3 and Advanced Camera for Surveys (ACS) were used in the creation of these HST images."

(PhysOrg.com) -- University of Michigan astronomers examined old galaxies and were surprised to discover that they are still making new stars. The results provide insights into how galaxies evolve with time.

U-M research fellow Alyson Ford and astronomy professor Joel Bregman are scheduled to present their findings May 31 at a meeting of the Canadian Astronomical Society in London, Ontario.

"Scientists thought these were dead galaxies that had finished making stars a long time ago," Ford said. "But we've shown that they are still alive and are forming stars at a fairly low level."

Galaxies generally come in two types: spiral galaxies, like our own Milky Way, and elliptical galaxies. The stars in spiral galaxies lie in a disk that also contains cold, dense gas, from which new stars are regularly formed at a rate of about one Sun per year.

Stars in elliptical galaxies, on the other hand, are nearly all billions of years old. These galaxies contain stars that orbit every which way, like bees around a beehive. Ellipticals have little, if any, cold gas, and no star formation was known.

"Astronomers previously studied star formation by looking at all of the light from an elliptical galaxy at once, because we usually can't see individual stars," Ford said. "Our trick is to make sensitive ultraviolet images with the Hubble Space Telescope, which allows us to see individual stars."

The technique enabled the astronomers to observe star formation, even if it is as little as one Sun every 100,000 years.

Ford and Bregman are working to understand the stellar birth rate and likelihood of stars forming in groups within ellipticals. In the Milky Way, stars usually form in associations containing from tens to 100,000 stars. In elliptical galaxies, conditions are different because there is no disk of cold material to form stars.

"We were confused by some of the colors of objects in our images until we realized that they must be star clusters, so most of the star formation happens in associations," Ford said.

The team's breakthrough came when they observed Messier 105, a normal elliptical galaxy that is 34 million light-years away, in the constellation Leo. Though there had been no previous indication of star formation in Messier 105, Ford and Bregman saw a few bright, very blue stars, resembling a single star 10 to 20 times the mass of the Sun.

They also saw objects that aren't blue enough to be single stars, but instead are clusters of many stars. When accounting for these clusters, stars are forming in Messier 105 at an average rate of one Sun every 10,000 years, Ford and Bregman concluded. "This is not just a burst of star formation but a continuous process," Ford said.

These findings raise new mysteries, such as the origin of the gas that forms the stars.

"We're at the beginning of a new line of research, which is very exciting, but at times confusing," Bregman said. "We hope to follow up this discovery with new observations that will really give us insight into the process of star formation in these 'dead' galaxies."

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Easy. Giant ellipticals likely producing new stars at high rate especially near their obscured cores. The huge core star at the galactic center producing and ejecting new matter at high rate to seed these new stars. This is the final stage of galactic evolution. Just a matter of time before this is acknowledged.

While some people indulge in giving answers to unasked questions I prefer to ask unanswered questions.

Why are some galaxies called "old"? Usually to be old means to be born earlier than those who are not old. Consequently, an old galaxy should be a galaxy that has come into existence earlier in time than other, "newer", galaxies. This scenario, however, doesn't fit quite well with the predominant lambda cold dark matter hypothesis according to which the first generation of galaxies has been seeded by primordial fluctuations and all galaxies thereafter are only products of collisions and mergers.

Indeed, it is a continuous process of predominately OLD blue giants producing new matter in their cores in a continuous process. Still the grey 'black' hole at the core will still produce the vast majority of the new matter. New line of research? Just read LaViolette's 'SubQuantum Kinectics'. Why reinvent the wheel?